1. Field of the Invention
The present invention concerns a process for the production of bimodal linear low-density polyethylene compositions. In addition, the present invention concerns bimodal linear low-density polyethylene compositions and films prepared thereof. In particular, the present invention relates to bimodal films having an improved balance between the mechanical properties and proccessability.
2. Description of Related Art
The proccessability on a film blowing line and the physical properties of the final film depend largely on the polymer structure, especially on the molecular weight distribution (MWD). If the polymer is bimodal, i.e., if the MWD is broad, the polymer can be expected to exhibit a good proccessability. Other important properties, which naturally depend on the application the polymer material is used in, comprise mechanical properties.
Films prepared from linear low-density polyethylene (PE-LLD) having a bimodal molecular weight distribution are known from prior art. The polymers are usually produced by polymerisation of ethylene and alpha-olefin comonomers in the presence of a polymerisation catalyst in two cascaded reactors.
Such films and methods of their preparation are known, e.g. from EP-A-692515, EP-A-691367, EP-A-773257, EP-A-57891 and WO-A-9618662.
EP-A-692515 discloses a film extruded from in-situ blend of polymers. The examples show that the low molecular weight component has an MFR2 of about 400 g/10 min and density of about 945 kg/m3. The high molecular weight component has an MFR21 of 4.4-5.4 g/10 min and density 901-902 kg/m3. The final polymer has MFR2 of 0.7-1.3 g/10 min, density of 926-929 kg/m3 and split 47/53-45/55. The polymerisation is conducted in two cascaded gas phase reactors. The material is especially useful in biaxially oriented films.
EP-A-691367 discloses films extruded from bimodal linear low-density polyethylene. The low molecular weight component has an MFR2 of 500-700 g/10 min and density of 929-934 kg/m3. The final polymer has MFR21 of 8-14 g/10 min, a density of 918-926 kg/m3 and a split of 40/60-36/64. The resulting films have a low blocking tendency, good proccessability and good mechanical properties (especially dart drop).
EP-A-773257 discloses shrink films made of in-situ blends of ethylene polymers. The blend has MFR2 of 0.7 g/10 min, FRR21/2 of 98 and density of 922 kg/m3. No data was given for low molecular weight and high molecular weight components. Split was 53/47.
EP-A-57891 discloses ethylene polymer compositions suitable for making films. The materials have a melt flow rate MFR2 of 0.2 to 10 g/10 min and density of 916 to 929 kg/m3. The films were reported to have a good mechanical strength. The polymer compositions were produced by melt blending, cascade slurry polymerisation or cascade solution polymerisation.
WO-A-9618662 discloses a process to produce ethylene polymers, by polymerising in a cascade of a prepolymerisation reactor, a loop reactor and a gas phase reactor. The resulting bimodal polymer had a melt flow rate MFR21 of 14 to 17 g/10 min and density of 923 to 925 kg/m3.
While the above documents disclose different film materials and manufacturing processes, a process for preparing a bimodal linear low-density polyethylene material suitable for film purposes, which material has good flow properties and good mechanical strength together with a good visual appearance, and which process is able to operate a long time in a stable manner is desired.